RFR: 8314488: Compiling the JDK with C++17

[Kim Barrett]

Please review this change to use C++17 for building C++ parts of the JDK. In
particular this affects HotSpot. This change also includes an update to the
HotSpot Style Guide regarding C++17 features and their use in HotSpot code.

Testing: mach5 tier1-8

This change includes a modification of the Style Guide. Rough consensus among
the HotSpot Group members is required to make such a change. Only Group
members should vote for approval (via the github PR), though reasoned
objections or comments from anyone will be considered. A decision on this
proposal will not be made before Friday 5-September-2025 at 12h00 UTC.

Since we're piggybacking on github PRs here, please use the PR review process
to approve (click on Review Changes > Approve), rather than sending a "vote:
yes" email reply that would be normal for a CFV.

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Commit messages:
 - fix a couple trailing whitespace issues
 - update style guide for c++17
 - build with c++17

Changes: https://git.openjdk.org/jdk/pull/26884/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26884&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314488
  Stats: 1183 lines in 7 files changed: 1097 ins; 35 del; 51 mod
  Patch: https://git.openjdk.org/jdk/pull/26884.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26884/head:pull/26884

PR: https://git.openjdk.org/jdk/pull/26884